Differential pressure transducers are commonly connected to a differential pressure source via a three-valve assembly. The three-valve assembly has to implement three functions. Firstly, it blocks the supply of the differential pressure, to permit the replacement of a faulty differential pressure transducer, for example; secondly, the three-valve assembly ensures that when the plant is put into operation, the usually very high static pressure is not applied to only one inlet of the differential pressure transducer, particularly of its measuring cell; and thirdly, the three-valve assembly serves to implement a short circuit by applying the same pressure to both inlets of the differential pressure transducer so that zero adjustment can be performed.
The commonly used three-valve assemblies have two inlet-end connections for connection to the differential pressure source and two outlet-end connections for connection to the differential pressure transducer. The two outlet-end connections are connected with one another via a short-circuit passage. Such a three-valve assembly contains three plain valves: A first inlet valve and a second inlet valve close the two inlet-end connections, and a third valve is disposed in the short-circuit passage.
A disadvantage of this three-valve assembly is that when the plant is put into operation, the three valves must be actuated in a particular sequence, for only if the third valve, i.e., the one in the short-circuit passage, is opened first will it be ensured that when one of the two inlet valves is opened, the pressure present at the respective inlet-end connection will be applied simultaneously to the two inlets of the differential pressure transducer. After both inlet valves have been opened, the valve in the short-circuit passage is closed and the measuring condition established.
If, as a result of an operating error, one of the two inlet valves is opened first, the full static pressure will be applied to only one inlet of the differential pressure transducer, in which case the transducer will not be destroyed only if protected by a highly efficient overload protection system. In the latter case, a disadvantage remains in that a (renewed) zero adjustment is generally inevitable.
In many practical applications, the requirement of sufficient overload protection for the differential pressure transducer can be fully met by the overload protection of the measuring cell. In those cases, the general requirement of increased overload protection, namely up to the full static pressure, is solely based on the above-explained errors in the operation of the three-valve assembly. Therefore, complicated and expensive overload protection devices are required, which unnecessarily add to the cost of the differential pressure transducer.